The present invention relates to improvements in apparatus for trimming the edges of stacked paper sheets or analogous commodities. More particularly, the invention relates to improvements in trimming apparatus for brochures, pamphlets, magazines, signatures, books or the like.
Commonly owned U.S. Pat. No. 4,202,232 (the disclosure of this patent is incorporated herein by reference) describes a trimming apparatus wherein several knives are movable between raised and lowered positions to trim three sides of a stack of sheets (e.g., a brochure) which is delivered to a trimming station. The arrangement is such that the operation of the conveyor which delivers successive stacks to the trimming station is synchronized with the working strokes of the knives so that the knives descend after the stack is properly positioned at the trimming station. Each knife descends toward and penetrates into the material of an anvil block which is provided therefor at the trimming station. Since the path of a knife during and after trimming of the stack at the trimming station is not identical with the path along which the knife advances in the absence of a commodity at the trimming station, the cutting edge of each knife enters a first portion of the respective block upon completion of a trimming operation and a different second portion of such block when the knife performs a working stroke without carrying out a trimming operation. This will be readily appreciated since the material of the stack offers a pronounced resistance to penetration of the cutting edge and invariably alters the path of the cutting edge by exerting upon the knife a lateral pressure which is the cause of penetration of the cutting edge into a portion of the block other than the portion which is penetrated into if the knife descends while the trimming station is unoccupied. The just discussed mode of operation entails the making of several grooves in the material of each anvil block with attendant pronounced shortening of the useful life of the blocks.
Heretofore known proposals to prevent unnecessary penetration of the cutting edges of trimming knives into the associated blocks in the absence of stacks of paper sheets or the like at the trimming station include the provision of means for arresting the knives if the trimming station is unoccupied due to the failure of the conveyor to deliver a stack of sheets in good time before the knives begin their working strokes. However, frequent stoppages and repeated starting of the trimming apparatus entail pronounced wear on the parts of such apparatus and cause a pronounced reduction of the output. In other words, it is desirable to keep the knives and other mobile parts of the trimming apparatus in motion even at a time when the trimming station fails to receive successive commodities at anticipated intervals.